Stripe DATEV export – automated.You've built Stripe in. You don't have to code your accounting too.

// app/api/checkout/session/route.ts
import Stripe from "stripe";

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!, { apiVersion: "2023-10-16" });

export async function POST(req: Request) {
  const { email, productId, priceId, customerId } = await req.json();

  // Falls nur productId vorhanden → zu Price (recurring) auflösen
  let resolvedPriceId = priceId;
  if (!resolvedPriceId && productId) {
    const prices = await stripe.prices.list({ product: productId, active: true, limit: 10 });
    resolvedPriceId = prices.data.find(p => p.recurring)?.id || prices.data[0]?.id;
  }
  if (!resolvedPriceId) {
    return Response.json({ error: "Kein Price gefunden." }, { status: 400 });
  }

  // Customer wiederverwenden/erstellen
  const customer = customerId ? customerId : (await stripe.customers.create({ email })).id;

  const session = await stripe.checkout.sessions.create({
    mode: "subscription",
    customer,
    line_items: [{ price: resolvedPriceId, quantity: 1 }],
    success_url: `${process.env.NEXT_PUBLIC_APP_URL}/success?session_id={CHECKOUT_SESSION_ID}`,
    cancel_url: `${process.env.NEXT_PUBLIC_APP_URL}/cancel`,
    allow_promotion_codes: true,
    metadata: { source: "pre-landing", plan_hint: productId || "n/a" },
  });

  return Response.json({ sessionId: session.id });
}

// app/(marketing)/subscribe/page.tsx
"use client";
import { loadStripe } from "@stripe/stripe-js";
import { useState } from "react";

const stripePromise = loadStripe(process.env.NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY!);

export default function SubscribePage() {
  const [loading, setLoading] = useState(false);

  async function handleSubscribe() {
    setLoading(true);
    const res = await fetch("/api/checkout/session", {
      method: "POST",
      body: JSON.stringify({
        email: "kunde@example.com",
        // eines von beiden:
        productId: "prod_ABC123", // → Server löst zu recurring price auf
        // priceId: "price_123",
      }),
    });

    const { sessionId } = await res.json();
    const stripe = await stripePromise;
    await stripe?.redirectToCheckout({ sessionId });
    setLoading(false);
  }

  return (
    <button className="btn btn-primary" onClick={handleSubscribe} disabled={loading}>
      {loading ? "Weiterleiten…" : "Abo starten"}
    </button>
  );
}

// app/api/stripe/webhook/route.ts
import Stripe from "stripe";

export const config = { api: { bodyParser: false } }; // nur Pages Router; App Router liefert raw body

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!, { apiVersion: "2023-10-16" });

export async function POST(req: Request) {
  const sig = req.headers.get("stripe-signature")!;
  const raw = await req.text();

  let event: Stripe.Event;
  try {
    event = stripe.webhooks.constructEvent(raw, sig, process.env.STRIPE_WEBHOOK_SECRET!);
  } catch (err: any) {
    return new Response(`Webhook Error: ${err.message}`, { status: 400 });
  }

  if (event.type === "checkout.session.completed") {
    const session = event.data.object as Stripe.Checkout.Session;
    // TODO: Onboarding, Feature-Freischaltung …
  }

  if (event.type === "invoice.paid") {
    const invoice = event.data.object as Stripe.Invoice;
    // TODO: Rechnungsdaten für finHero/DATEV vorbereiten
  }

  return new Response("ok");
}